The need to transmit signals (e.g., RF, acoustic or seismic signals) back towards a transmitting source (such as a wireless or RF device/system, a satellite device/system, an acoustic device/system, a seismic device/system and the like) in a precise manner arises in many communication systems and applications. When knowing where exactly the transmitting source is located, it can be assured that the transmitted signal reaches said source. This issue can be especially important, when communicating with distant sources, such as satellites, and/or when the accuracy in transmitting the signal in a precise direction is important.
The problem of transmitting RF signals back towards the transmitting source has been recognized in the prior art, and various systems have been developed to provide a solution, disclosing various time-reversal and phase-conjugation techniques. For example, a prior art method called “heterodyning” or “heterodyne mixing” is a phase conjugation method, according to which a signal received at each antenna is multiplied by means of a local oscillator, having a frequency which is two times of the frequency of the received signal; the signal is then filtered via low pass filter (LPF). The method may handle both far as well as near-field targets, and can be implemented in various antenna array geometries. Heterodyne mixing is a narrowband method that may handle only signals of known frequencies; otherwise, it requires frequency estimation of the received signals. Furthermore, the heterodyne mixing method cannot handle simultaneous signals.
In addition, there are several prior art systems, such as phased arrays. For using such arrays, determining an angular direction of the transmitting source is required, as well as also calculating the phase differences of the signal to be transmitted towards the source from each antenna of said phased array. It should be noted that calculating the signal phase differences requires relatively complex processing, and also the usage of a phase shifter is required. In addition, the accuracy in calculating the phase differences of the signal depends on how accurately an angular direction of the source is determined. Often, prior to being received by means of antennas, a RF signal is reflected from several accidental reflectors, such as tower blocks, balconies and the like. Thus, measurements of the source angular direction depend on such accidental factors, which in turn lead to receiving incorrect results. Further, for improving the measurements when operating at relatively high frequencies, e.g., above 2 GHz (GigaHertz), the antennas have to be positioned relatively close to one another, which may cause juxtaposition of antennas within said phased array (especially, when receiving RF signals), and can also cause antennas to overheat during RF signals transmission. Moreover, this can result in disturbances in calculating phase differences of the signal to be transmitted from each antenna. Usually, the antennas within the phased array have to be positioned on a straight plane in order to be able to further determine and calculate the required angular direction of the signal to be transmitted by means of each antenna (in order to get the greatest overall ERP (Effective Radiated Power) in the direction to which the RF signal is transmitted).
For example, U.S. Pat. No. 4,383,332 relates to a mobile radio base station capable of communicating with a large number of mobile stations by implementing space diversity and time-division retransmission techniques in a digital communication system. The digital base station contains a plurality of antenna elements and a plurality of retransmission branches associated in a one-to-one relationship. When the base station transmits a signal back to the mobile station, each retransmission branch adds the conjugate of its associated random phase to the signal to be transmitted, allowing the environment to “undo” the effect of the conjugate random phase so that the signals transmitted by the plurality of antenna elements will arrive coherently at the mobile station. According to U.S. Pat. No. 4,383,332, the retro-directivity is achieved by analog implementation of the phase conjugation for transmitting narrowband signals with known frequencies, each narrowband signal at the same time.
Further, U.S. Pat. No. 6,630,905 discloses a system and method for automatically generating a return beam in the direction of a received beam. The system includes a phased array antenna for receiving a radio frequency signal having a first wavefront from a first direction. In response to this signal, the second signal is provided having a second wavefront. The second signal is a phase conjugate of the first signal and is transmitted in a reverse direction relative to the direction of the first signal.
Also, according to the prior art, a corner reflector can be used for reflecting electromagnetic waves back towards the transmitting source. The RF wave hits the surface of the corner reflector, and due to its unique structure, the wave is reflected back towards the source. The corner reflector can be used for communication applications, where signals have to be immediately reflected back to the source. For example, by placing such a reflector on the Moon, this can help measure the Moon's orbit in a more accurate manner. According to U.S. Pat. No. 5,909,299, a detailed mapping of the magnetosphere is made possible by deploying hundreds of attitude-impervious micro-satellites, in the form of small corner reflectors with piezoelectric mirror surfaces, from a single mother satellite at spacings of as little as 1 km in equatorial and elliptical orbits. The micro-satellites carry magneto-sensors whose output is transmitted to a ground station by modulating the reflection of a laser beam transmitted to the micro-satellite by the ground station. U.S. Pat. No. 5,909,299 relates to transmitting narrowband signals of a known frequency, each narrowband signal at the same time.
In addition, the prior art teaches about a Van Atta reflector array (U.S. Pat. No. 2,908,002) that is an array, in which elements are interconnected to reradiate received energy back in the direction of arrival. The Van Atta reflector array requires ULA (Uniform Linear Array) geometry and can handle signals received from far sources to ensure planar wavefronts.
There is a continues need in the prior art to provide a digital retro-directive method and system configured to enable transmitting a signal from a plurality of antennas towards a transmitting source in a substantially simultaneous (synchronous) manner and in a substantially precise angular direction, without the need to measure such direction. Also, there is a need to provide a retro-directive method and system that enables simultaneous transmitting of signals from each antenna back towards a transmitting source without calculating signals phase differences, and further enables transmitting signals having unknown frequencies. In addition, there is a need to provide a retro-directive method and system, wherein there is no need in positioning antennas on a straight plane for enabling said simultaneous transmitting.
Moreover, there is a need to provide a retro-directive method being applicable for both narrow and wideband communication systems, further enabling simultaneous multi-signal communication with multiple far-distanced and/or near-field transmitting sources.